The present invention relates to fine powders of PTFE having a very good thermal stability, particularly suitable for lubricated extrusion processes with high reduction ratios.
Specifically, the invention relates to fine powders of modified PTFE having a core-shell structure wherein the core is formed by PTFE modified by a perfluorodioxole monomer or which cyclizes during the polymerization to give a ring containing at least one oxygen atom, and wherein the shell is formed by PTFE modified by perfluoropropene. The fine powders of the modified PTFE of the present invention show very good thermal stability and are processable by lubricated extrusion at high reduction ratios at low extrusion pressure. This treatment allows to obtain manufactured articles without surface defects, i.e. without fractures and roughnesses.
It is known that the modified PTFE is a polymer based on tetrafluoroethylene (TFE) which contains small amounts of comonomers and which, as well as the PTFE homopolymer, cannot be transformed as thermoprocessable.
The fine PTFE powders are obtained by the polymerization in dispersion (emulsion). In said process a sufficiently high amount of surfactant is used so to stabilize the colloidal PTFE particles and a mild stirring is applied to avoid the polymer coagulation (precipitation). Subsequently the latex obtained from said process is coagulated, and the powder obtained from the coagulation is called xe2x80x9cfine poderxe2x80x9d.
Another TFE polymerization process is known: the suspension polymerization, wherein only a small or no amount of surfactant is used and a strong stirring is applied to obtain the precipitated polymer flocks. With this process in suspension fine powders are not obtained.
The PTFE fine powders are transformed by the lubricated extrusion process for obtaining the desired manufactured articles. To obtain a productivity increase and manufactured articles having thin wall, for example used in the cable insulation, it is important to be able to extrude at high reduction ratios. With the term reduction rate (RR) the A2:A1 ratio is meant, wherein:
A2 is the cyclinder section surface in which the PTFE preform is introduced before the extrusion,
A1 is the outlet section surface of the extruder nozzle.
Generally, by increasing the reduction ratio the extrusion pressure increases and therefore the defects of the extruded manufactured article increase. Therefore it is important to obtain a fine PTFE powder which is suitable to be extruded at high reduction ratios with a sufficiently low pressure obtaining extruded manufactured articles which do not show surface defects, such as for example fractures, roughnesses.
The PTFE manufactured articles are often used under extreme conditions, including applications at high temperature, wherein it is important to have a material having the highest thermal stability as possible. Furthermore, a high thermal stability of the PTFE is required since the PTFE has not to undergo thermal degradation during the transformation process.
In U.S. Pat. No. 4,036,802 it is described how to obtain fine PTFE powders suitable for the transformation by lubricated extrusion at high reduction ratios (RR), obtaining in the polymerization process a core-shell structure, wherein both the core and the shell of the particle are formed by PTFE modified with the same comonomer, and the content of the modifying agent comonomer in the core is higher than that of the shell. Preferably perfluoroalkylvinylethers are used as modifying comonomers. The process described in this patent requires, after the polymerization step of the core, a partial removal of the TFE and of the comonomer from the reactor, the stop of the polymerization, the subsequent repressurization by using only TFE, and the subsequent restarting of tile polymerization. Such process, called xe2x80x9cvent-repressurexe2x80x9d, has the purpose to reduce the comonomer amount in the particle shell with respect to the core. However, the reaction stop and restarting very often cause latex destabilization phenomena which cause the coagulum formation both in the reactor and during the subsequent latex processing (transfers, storage, filtering, concentration, etc.). Besides such process complicates the productive process and it reduces the productivity of the polymerization reactor.
In U.S. Pat. No. 5,731,394 it is described how to obtain modified fine powders suitable for the extrusion at high reduction ratios without the vent-pressure procedure, using the combination of perfluoroalkylethylene and perfluoropropylvinylether for the core modification. In this patent the obtained fine powders show a low extrusion pressure and a good adhesion to the cable. The low extrusion pressure is obtained by using a shell based on PTFE having a low molecular weight. However, the use of PTFE having a low molecular weight implies a low thermal stability of the obtained fine powder. Tests carried out by the Applicant, see the Examples, show that the presence of perfluorobutylethylene as modifying agent in the core decreases the thermal stability of the obtained fine powder.
In EP 764,668 it is described how to obtain modified fine powders suitable for high reduction ratios, preparing the core with PTFE modified by the perfluorobutylethylene (PFBE), while the shell is modified by perfluoropropene and applying the vent-repressure procedure for a complet removal of PFBE and TFE after having polymerized the core. Tests carried out by the Applicant, see the Examples, wherein perfluorobutylethylene is used for the core modification, give a polymer having a low thermal stability. Besides, the vent-repressure procedure used in EP 764,668 has an impact on the stability of the obtained latex and on the reactor productivity as above said.
In U.S. Pat. No. 4,391,940 modified fine powders suitable for the extrusion at high reduction ratios are obtained, making a three-layer core-shell structure of the particle: the core is formed by PTFE modified with at least a comonomer, the intermediate layer is PTFE homopolymer, and the particle shell is PTFE modified by fluoroolefins; the ratio by weight between the intermediate layer and the shell being in the range 75:25-99.5:0.5, the core being from 5 to 20% by weight of the total particle. To obtain said structure two techniques are used: vent-repressure and seed. The use of these techniques complicates the process, lowers the stability of the latex and reduces the reactor productivity.
In EP 380,120 fine powders having a core-shell structure are obtained, wherein the core is modified by a perfluorodioxole, the shell being formed by PTFE homopolymer or PTFE modified with a perfluoroalkylvinylether. The fine powders obtained according to this patent are not suitable for the extrusion at high reduction ratios. In fact, the manufactured articles obtained by extrusion at high reduction ratios of fine powders having a core modified by a perfluorodioxole and the shell modified by perfluoropropylvinylether show a rough surface (see the Examples).
The need was therefore felt to have available fine powders of modified PTFE having an improved thermal stability, suitable to be transformed by lubricated extrusion at high reduction ratios with a low extrusion pressure for obtaining extruded manufactured articles not showing surface defects, for example fractures, roughnesses.
An object of the present invention are therefore fine powders of modified polytetrafluoroethylene (PTFE) having a core-shell structure; the fine powder being formed by:
1) a xe2x80x9ccorexe2x80x9d of tetrafluoroethylene (TFE) modified by one or more monomers selected from:
a) dioxoles of formula: 
wherein W1 and W2, equal to or different from each other, represent F or CF3, W4xe2x95x90F, Rf, ORf, with Rf equal to a perfluoroalkyl radical containing from 1 to 5 carbon atoms;
b) one or more perfluorinated monomers which cyclize during the polymerization;
2) a xe2x80x9cshellxe2x80x9d of TFE modified by perfluoropropene (PFP); the amount, referred to the core+shell total particle, of comonomers of 1) being in the range 0.01-0.06% by weight; the amount, referred to the core+shell total particle, of perfluoropropene of 2) being in the range 0.002-0.06% by weight; the thermal instability index (TII) of the fine powder being equal to or lower than 5, determined according to ASTM D4895.
The preparation of the comonomers dioxoles a) is reported for example in U.S. Pat. No. 4,908,461, U.S. Pat. No. 5,245,054, U.S. Pat. No. 5,296,617 and in EP 633,257.
The preferred comonomers dioxoles a) are:
the compound having W1xe2x95x90W2xe2x95x90F, W4xe2x95x90OCF3 (perfluoromethoxydioxole), the preparation of which is described in EP 633,257;
the compound having W1xe2x95x90W2xe2x95x90CF3, W4xe2x95x90F, the preparation of which is reported in U.S. Pat. No. 3,865,845 and U.S. Pat. No. 3,978,030;
the compound having W1xe2x95x90W2xe2x95x90W4xe2x95x90F, the preparation of which is reported in U.S. Pat. No. 3,978,030 and U.S. Pat. No. 3,865,845.
Among the perfluorinated comonomers of type b) it can be mentioned:
bisvinyloxymethanes of structure:
CF2xe2x95x90CFxe2x80x94Oxe2x80x94CX3X4xe2x80x94Oxe2x80x94CFxe2x95x90CF2, 
wherein X3 and X4, equal to or different from each other, are F or CF3, which form in the polymer cyclic repeating units having the structure: 
Preferably perfluorobisvinyloxymethane is used wherein X3xe2x95x90X4xe2x95x90F. Bisvinyloxymethanes are described in U.S. Pat. No. 5,589,557.
perfluorinated compounds having the structure: 
Said compounds are described in U.S. Pat. No. 4,910,276.
The modified PTFE fine powders of the present invention contain therefore a very small amount of comonomers, such that the polymer is not thermoprocessable from the melt, i.e. the melt polymer viscosity is higher than 108 Paxc3x97sec at 380xc2x0 C. The type and the amount of comonomers incorporated in the fine powder of the present invention guarantee a very good thermal stability, represented by a thermal instability index (TII) equal to or lower than 5, preferably lower than 1. This low value of thermal instability represents a clear improvement with respect to the fine powders of the prior art, wherein TII indexes generally lower than 20 are reported.
The fine powders of the present invention can be extruded at high reduction ratios, higher than 1:1,600, for example 1:2,500. If desired, the powders of the invention can also be processed at low reduction ratios (for example RR=1:115).
The molecular weight of the modified PTFE of the present invention is high as shown from the absolute specific weight values, which are of the order of 2.1-2.2 g/cm3. The skilled is able to estimate from the density values and from the comonomer content the molecular weight of the modified PTFE.
It has been found by the Applicant that when the content of comonomer 1) in the core is lower than 0.01% by weight, the obtained powder generates in the extrusion at high reduction ratios pipes having a poor consistence, wherefore the material separates in more pieces during the extrusion or fractured pipes are obtained. When on the contrary the content is higher than. 0.06% by weight, the extrusion pressure increases too much and the obtained product cannot be extruded with high reduction ratios. Besides, when the comonomer 1) amount is higher than the indicated range, a further drawback resides in that a part of the unreacted comonomer 1) remains in the reactor during the polymerization of the shell. When, as in this case, the shell contains PTFE modified both by the comonomer 1) and by the perfluoropropene 2), no extruded manufactured articles having a good surface finishing coat are obtained from the extrusion process, wherefore the surface shows defects which make the manufactured article unsuitable to the use.
It has been found by the Applicant that when the perfluoropropene content in the particle shell is lower than the indicated values, the extrusion pressure of the obtained fine powders strongly increases and the fine powder cannot be processed at high reduction ratios. On the contrary, when the PFP content is higher than the indicated values, the manufactured article has no suitable tensile properties before sintering, in particular it shows low green strength, wherefore the sintered manufactured article shows undesired surface defects.
A further object of the invention is a process for obtaining the above PTFE fine powders, comprising:
radical emulsion polymerization of tetrafluoroethylene (TFE) with a comonomer 1), for example perfluoromethoxydioxole, wherein the comonomer 1) is fed into the reactor during the initial part of the reaction; in particular, all the comonomer 1) is fed into the reactor before 20% of the TFE conversion, preferably all the comonomer 1) is fed at the starting of the reaction;
feeding of pe:rfluoropropene (PFP) after more than 80% of tetrafluoroethylene which is to be polymerized, has reacted, preferably after not more than 95% of TFE has reacted; preferably the feeding of the desired PFP amount is carried out in a continuous way until the end of the TFE polymerization; during the polymerization the TFE pressure being maintained constant at a prefixed value between 10 and 30 bar.
Therefore, in the process of the invention the vent-re-pressure system is not used, i.e. a reduction of the reaction pressure through the reactor vent and a subsequent restoration of the pressure during the reaction is avoided.
The polymerization is carried out at temperatures in the range 50xc2x0 C.-100xc2x0 C., preferably 70xc2x0 C.-90xc2x0 C. The combination of the polymerization temperature and of the amount of polymerization initiators, must be such as to assure a high molecular weight of the obtained polymer, as above defined by the specific weight values. The skilled on the basis of the temperature used for the polymerization, is able to identify the initiator amount for obtaining said specific weight values of the polymer.
The fed amount of comonomer 1) is generally in the range 0.01-0.06% by weight based on the total reacted TFE, so that the content of comonomer 1) in the polymer is in the range 0.01 and 0.06% by weight.
As regards perfluoropropene (PFP), the feeding into the reactor must start after more than 80% of tetrafluoroethylene has reacted and before 95% of TFE has reacted. In any case the comonomer 1) must not be present any longer in the reactor when PFP is fed. Preferably, the PFP feeding is carried out in a continuous way until the end of the polymerization. In this way the reaction temperature is better controlled avoiding sudden temperature drops.
In the process of the present invention as radical initiators those typical of the TFE polymerization are used. Preferably the persulphate, permanganate salts or hydrosoluble organic peroxides, such as for example the disuccinic acid peroxide (DSAP) are used. More preferably a mixture of the disuccinic acid peroxide (DSAP) with a persulphate, preferably ammonium persulphate (APS), is used as initiator, with a ratio by weight DSAP/APS in the range 2-50, preferably 10-25. Generally an amount of one or more initiators in the range 3 ppm-350 ppm by weight with respect. to the polymerization water, preferably an amount in the range 15 ppm-150 ppm by weight is used. Optionally the initiators can be used together with a reducing agent for producing a part of radicals at the starting of the reaction by a Red-Ox reaction. As reducing agent the Mohr (SdM) salt (NH4)2Fe(SO4)26H2O can be used. Generally, the amount of the reducing agent is lower than 50% molar referred to the initiator, preferably lower than 25 molar.
As said, the polymerization is carried. out under a TFE pressure in the range 10-30 bar. Generally, during the polymerization the pressure is maintained constant by feeding the TFE so as to compensate for the reacted monomer. It can be advantageous in the process of the present invention to carry out a partial react-down at the reaction end. When the polymerization reaches the final TFE conversion, the TFE and PFP feedings are stopped, temperature and stirring rate are maintained unchanged and the reaction is continued until the pressure inside the reactor falls to the desired pressure, for example 5 bar if the initial polymerization pressure is 20 bar. Then, the stirring is stopped and the reactor is evacuated and cooled. Said procedure allows to form an external layer of the shell having a higher perfluoropropene content.
The average size of the latex particles is substantially in connection with the comonomer 1) content in the core and with the initial concentration of the fluorinated surfactant defined further on. By increasing the diameter of the latex particles, the extrusion pressure tendentially decreases. However, when the particles become too large, the colloidal stability of the latex strongly decreases causing coagulum formation. Consequently, both macroaggregates, in the form of coagulum blocks inside the reactor and microaggregates, which remain dispersed in the latex, form; they after the transformation cause defects of the manufactured article extruded at high reduction ratios.
For said reason the average diameter of the latex particles according to the present invention must be in the range 150 nm-400 nm, preferably 220 nm-300 nm. In order to control the latex particle diameter, the addition of metal cations can be used at the starting of the polymerization according to U.S. Pat. No. 3,526,614, for example under the form of zinc chloride. Another useful method for the particle diameter control is the distributed addition of the fluorinated suri-actant during the polymerization, for example according to U.S. Pat. No. 3,009,892.
A further advantage of the invention process is that latex particles having a substantially spherical shape are obtained. The analysis at the scanning electronic microscope (SEM) shows that the latex particles obtained according to the present invention are substantially spherical. This is advantageous since it allows to furtherly reduce the extrusion pressure of the fine powders obtained from latexes.
The fluorinated surfactants used in emulsion in the polymerization process of the present invention can be selected from the following:
T-O-(C3F6O)n(CF2O)mCF2COOMxe2x80x83xe2x80x83(IA) 
wherein: Mxe2x95x90H, NH4, Na, Li, K and n can range from 1 to 6,
m/n is in the range 0.05-0.1
T is a (per)fluoroalkyl group selected from: xe2x80x94CF3,
xe2x80x94C2F5, xe2x80x94C3F7, xe2x80x94CF2Cl, xe2x80x94C2F4Cl, xe2x80x94C3F6Cl; optionally one or two F atoms can be substituted by H;
CF3(CF2)8COOMxe2x80x83xe2x80x83(IIA) 
wherein n can range from 4 to 12
Fxe2x80x94(CF2xe2x80x94CF2)nxe2x80x94CH2xe2x80x94CH2xe2x80x94SO3Mxe2x80x83xe2x80x83(IIIA) 
wherein Mxe2x95x90H, NH4Na, Li, K and r can range from 2 to 5.
Among the surfactants (IIIA), preferably C6F13CH2CH2SO3M wherein Mxe2x95x90H or monovalent cation, for example K (commercial product ZONYL(copyright) 6,2-TBS or FORAFAC(copyright) 1033) can be mentioned.
The preferred surfactants are the perfluorooctanoic acid salts, in particular ammonium perfluorooctanoate (PFOA). The total surfactant. amount is generally in the range 0.5-5 g for liter of reaction medium, preferably 1-3 g for liter. The fluorinated surfactant can also be partly fed at the starting of the polymerization and partly during the polymerization reaction.
To prevent the aggregation phenomena of the latex particles during the polymerization it is advantageous to add into the reactor a stabilizer. For the process of the present invention paraffins with softening point in the range 48xc2x0 C.-62xc2x0 C. are preferred as stabilizers.
The latex obtained from the polymerization is commonly diluted until a concentration of 10-15% by weight of polymer and is coagulated under mechanical stirring in the presence of an electrolyte at a concentration in the range 10xe2x88x923-10xe2x88x921 moles/l. The coagulation conditions (stirring rate and temperature) are selected so as to obtain a fine powder having an average size in the range 350-600 microns. Subsequently the fine powders obtained from the coagulation are separated from the water and dried in an oven at a temperature in the range 105xc2x0 C.-160xc2x0 C. It is known that the drying temperature affects the extrusion pressure: by increasing the drying temperature, the extrusion pressure increases, therefore in the process of the present invention it is preferable that the fine powders are dried at a temperature in the range 110xc2x0-140xc2x0 C. During the post-treatment process (coagulation and drying) it is very important that the fine PTFE powders are not damaged or crushed, since if the granules are damaged, it is not possible to obtain an extruded manufactured article of good quality. Among manufactured articles, pipes and cable coatings can be mentioned.
Some embodyment Examples of the present invention are reported hereinafter, the purpose of which is merely illustrative but not limitative of the scope of the invention itself.